I think I agree with your assessment. This will not be Markstrom's year to assume a relevant tending role with Florida. The ever going Luongo concern simply amplifies this.
While you may try to trade him, I don't know if it will be worth your time as I'm guessing most of the GM's in your league feel the same way.
From the guys below...if you're looking for a potential impact player (Your F and D looks pretty solid), I'd focus on replacing him with another available Goaltender.
Of the guys below that you mentioned, I'd be most interested in:
Khudobin - With Thomas out of the picture, he's the definite back up to Rask. This is Rask's chance to shine but one thing we learned...sometimes the heir apparent stumbles in his first opportunity to sit in the drivers seat. While this isn't really Rask's first opportunity and he did Dazzle in his first, this is almost like a do-over and I have a gut feeling, he isn't as spick and span this season as he likely will be over his career. Plus...nagging injuries which Boston won't want to take a chance with on their Young Golden boy of the future. Khudobin hould very welll see plenty of ice time and excel at it (Consider Halak in Montreal...battling with Theodore and Price).
Failing that, I'd gamble on The Monster. This guy never lived up to his potential in Toronto...and with what should be a HUGE chip on his shoulder, has EVERYTHING to prove. He's now playing for a contender and with Howard in his way...I see a 70/30 split atleast...and a chance to shine in the event of injury.
The other 3 really do nothing for me.
Best of luck.